They are different programs based around the same engine. Firebird is in beta and has many bugs (including strange plugin problems) and is a standalone browser. Mozilla 1.6 is larger, contains a web editor and mail program, is a bit more mature, and doesn't crash as often (at least in Windows). The nice thing about Mozilla is although it crashes, it doesn't do it as often and doesn't take the computer down with it
Oh, and any program that has a version number that starts with 0 (as in 0.7 or 0.22) that usually (
usually) means the program in question is 'beta' or 'alpha' and will contain bugs (the later the version number, the lesser the bugs).